This invention relates to a method of treating lettuce for use in salads and the like to extend the storage life of the lettuce to about 4 weeks or more under refrigeration.
Lettuce is one of the most perishable of foods. To remain usable following harvesting, it normally requires refrigerated shipping and storage, preferably in a controlled atmosphere. In recent years much interest has developed in the preprocessing of the more common ingredients of salads, either mixed or kept separated until shortly before being served. This involves more or less cutting or other subdivision of the salad components, which has a tendency to hasten deterioration of the subdivided vegetables because of the release of enzymes and moisture from the tissues that are cut or otherwise damaged during subdivision of the vegetable. Much effort has been expended on the problem of extending the storage life of cut lettuce by slowing down its loss of too much of the crispness as well as attractive green color normally associated with and expected of lettuce in a salad. Once lettuce begins to deteriorate, it rapidly loses its desirability for salads or in sandwiches or for any other conventional food usage thereof. Numerous kinds of deterioration of lettuce can occur, such as wilting, bacterial growth, fungal growth, the natural reactions caused by enzymes present in the lettuce tissues, and the effects of oxidations. Deterioration probably begins as soon as a head of lettuce is cut from its stalk in the field. The problem then becomes one of slowing down as much as possible a variety of possible routes of deterioration any one of which, if allowed to take its usual natural course, will result in the necessity to discard the lettuce much sooner than one wishes to have occur.
Kubu, U.S. Pat. No. 3,849,581, discloses a method for treating shredded lettuce to prevent wilting and discoloration during storage by treating the shredded lettuce with an antioxidant solution, draining, and centrifuging to partially dehydrate the treated lettuce. Cummin et al., U.S. Pat. No. 3,795,749, discloses a method for extending the storage life of heads of lettuce by packaging the heads soon after harvesting in plasticized polyvinyl chloride film having at least three times as great permeability to carbon dioxide as to oxygen and a certain minimum permeability to carbon dioxide. Moisture vapor permeability of the film may also be important for allowing the escape of a desirable amount of, but not too much, moisture from the lettuce. The heads of lettuce may also be treated with antimicrobial agents, such as chlorine solutions. Various types and combinations of plastic films have been proposed for packaging fresh or raw vegetables, including lettuce, for example, Purcell et al., U.S. Pat. No. 3,423,212, Badran et al., U.S. Pat. No. 3,450,543, and Rumberger, U.S. Pat. No. 3,630,759. These methods have helped to greater or lesser degrees in staving off deterioration of various vegetables; but none has proven adequate for cut lettuce to render it capable of being stored in a refrigerated area for a fairly long time in this precut state while retaining its desirable characteristics after such long storage in cut form.
An article in Food Service Processing & Packaging, Vol. 34, No. 9, page 14, entitled "Preprocessed Salad Components -- Extend Life, Cut Volume Up To 65%" describes some of the preliminary research work which led to the invention described and claimed herein. However, the packaging of the chemically treated lettuce plays a very important part in determining the storage life of cut lettuce. Furthermore, the permeability of the packaging material with respect to oxygen, carbon dioxide, and moisture vapor is important for this purpose, as will be apparent as the description of the invention proceeds.
It is, therefore, an object of the invention to provide a method of treating cut lettuce to extend the storage life thereof under refrigeration.
Other objects and advantages will become apparent from the following description of the invention.